Ribbon support for high fidelity electroacoustical sound transducers



. 2552,31 1 HIGH FIDELITY ELECTRQACOUSTICAL SOUND TRANSDUCERS Fzlud Aug 28 1948 May 8, 1951 X R. K. DUNCAN RIBBON SUPPORT FOR INVENTOR HUBER-r KDUNEAN all; ATTORNE Patented May 8, 1951 RIBBON SUPPORT FOR HIGH FIDELITY ELECTROACOUSTICAL SOUND TRANS- DUCERS Robert K. Duncan, Mount Ephraim, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 28, 1948, Serial No. 46,648

11 Claims. 1

This invention relates to high fidelity electroacoustical sound transducers, and more particularly to high fidelity microphones of the velocity, or pressure gradient, responsive type. In high fidelity electro-acoustical apparatus, such as ribbon diaphragm microphones of the velocity, or pressure gradient, responsive type, it is desirable to provide therein for a uniform or flat, and extended, frequency response characteristic. Basically and fundamentally, however, microphones of this type should also provide for an identical frequency response characteristic on both sides of the ribbon diaphragm.

In order that a velocity, or pressure gradient, responsive microphone may be provided with an extended high frequency response, it is necessary that the sound Wave path length in traversing from one side of the ribbon diaphragm, around the pole structure, to the opposite side of the ribbon, be kept as short as possible, that is, it should be less than one quarter of the wavelength at the highest frequency at which reproduction is desired. The obvious and conventional way of reducing this sound wave path distance is to minimize the cross section area of the pole pieces and to provide a suitable support for the ribbon diaphragm. The ribbon support is customarily bolted, soldered, or otherwise fastened, to the pole piece ends or tips, and the structure provided may have no appreciable effect upon the frequency response characteristic in .the lower audio frequency range.

However, should the desired range of the microphone extend for example, to as high as 15,000 C. P. S., or higher, the wavelength'is so short that the front-to-back distance of the diaphragm is a considerable portion of a wavelength of the sound at that frequency. For 15,000 cycle response, the sound wave path length should be .225 inch or less, whereas it may be substantially one inch. This necessitates providing pole pieces having ends or tips which are extremely small in cross-sectional area, the dimensions being so small they are prohibitive for fastening the ribbon support or clamp to the ends of the pole pieces with screws. Even soldering the clamp to so small a surface is not feasible. Under such circumstances, it is customary to use a frame-like structure which is attached to the sides of the pole pieces by any suitable means, the structure having clamps at each end thereof, for suspending the ribbon in the air gap between the pole pieces.

7 High fidelity, prior art microphones having an extended high frequency range, make use of such a supporting structure for the ribbon element. However, examination of the acoustical circuit of microphones constructed in this manner, discloses the baffle area, which is presented to a sound wave approaching the microphone, to be different from one side than the other due to the presence of the ribbon support on only the one side of the microphone. This results in unequal frequency response characteristics from opposite sides of the microphone, which is objectionable from an efficient and high fidelity acoustical standpoint.

It is a primary object of this invention, therefore, to provide an improved acoustical supporting structure for eleotro-acoustical apparatus of the ribbon microphone type, which not only obvivides identical frequency response characteristics from both sides of the ribbon element.

It is another object of the invention, toprovide a symmetrical acoustical structure for velocity, or pressure gradient, responsive type microphones.

It is also an object of the invention, to provide an improved supporting structure for the ribbon element of velocity, or pressure gradient microphones, which is of relatively simple, low

cost construction, and which is easily assembled in manufacture.

In accordance with the present invention, there is provided a high fidelity microphone with a symmetrical supporting structure for the ribbon element. The supporting structure comprises a pair of frame-like plates symmetrically disposed on opposite sides of the pole pieces in such a manner that the microphone becomes symmetrical acoustically and equal frequency response characteristics are obtained from both sides .of the microphone.

The novel features that I consider characteristic of my invention, as well as additional objects and advantages thereof, will be better understood from the following description when read in connection with the accompanying drawe ing, in which:

Figure 1 is a front view, in elevation, and on an enlarged scale, of a high fidelity ribbon microphone embodying the invention, a portion being shown broken away to show details of the interior construction,

Figure 2 is a side view, partly in section, of the microphone of Figure 1, taken along the line 2-2,

Figure 3 is a top view of the microphone of Figure 1, also partly in section and on the same scale, taken along the line 3-3 of Figure 2, and

Figures 4 and 5 are graphs showing curves illustrating certain operating or response characteristics which are desirable for a further understanding of the invention.

Referring to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown, in Figures 1:, 2, and 3, a velocity, or pressure gradient, responsive microphone I provided with a ribbon diaphragm supporting structure 3.

The microphone comprises a pair of permanent magnets 5, I mounted in spaced relation on a base plate 9. A pair of elongated L-shaped pole pieces ll, l3 are mounted on the ends of the magnets opposite from the base plate 9. The assembly of magnets, base plate and pole pieces.- are securely held together by means of a pair of bolts I5. The pole pieces H, I3; include tapered portions ll, [9 having flat faces 2!, 23, the faces being arranged in spaced apart, parallel relation to provide an air gap 25.

An elongated, vibratile member such as a. conductive ribbon diaphragm 27, preferably transversely corrugated, of suitable electric conducting material, is suspended in the air gap for movement in response to the velocity component of sound waves impinging thereon from opposite sides thereof. ribbon element 21, comprises two flat, open, rectangular supporting frames 29, 3| of non-mag netic material having central openings 33, 35, the plates being mounted in planes parallel to and at equal distances from the opposite sides of the vibratile member 21. The plates 29, 3!- are also mounted with their adjacent ends connected so that the plates will be in superposed relation on opposite sides of the tapered portions i1, (9, of the pole pieces with the openings providing free access for sound waves toreach the air gap 25.

The plates 29, 3| are also of such length that their ends extend beyond the ends of the air gap. Disposed between the ends of the supporting frames are spacing members 37 which provide";

extensions forming jaws between which the ends of the ribbon element 2'! are clamped, bolts 39' being provided for maintaining the supporting structure in its assembled relation. This type of mounting permits adjustment of the vibratory member 2': within the air gap. The plates may be tapered or reduced in width adjacent one end to conform in shape with the pole pieces as shown by the curved portions 4:, 43 in plate 29. This construction serves to maintain the shortened sound wave path length provided by the ends of the pole pieces. In this respect the cut-out portions may be made to conform to the outline of any pole piece structure adjacent the ends thereof.

Both rectangular frames may also be covered with a suitable material such, for example, a the silk screen 4d, shown in Figure l, for providing the microphone with an acoustical damping means for low frequency sound waves in order to counteract the increased response due to ribbon resonance. In addition, this material will also serve to protect the air gap 25 against the infiltration of magnetic dirt particles.

Positioning or retaining grooves 45, 41 are provided on opposite sides of the tapered portions The supporting structure 3 for the I1, [9 of the pole pieces in which the marginal, longitudinal edges of the frames 29, 3| are received for mounting. At least one of the plates is securely fastened to the sides of the tapered portions, in the grooves provided therefor, by any suitable means, other than screws or bolts, such as soldering, welding or the equivalent thereof. The reason bolts or screws are not recommended that it would be necessary to provide apertures in the pole pieces in which the screws or bolts could be fastened. If apertures are made in the pole pieces, the uniformity in the flux distribution and the required flux density in the pole pieces in the region of the apertures may be disturbed, with a consequent loss in acoustical efficiency of the apparatus and possible impairment of the frequency response characteristic.

It will readily be understood, by those skilled in the art, that applying similar open supporting frames, symmetrically disposed, on opposite sides of the air gap, that the microphone becomes symmetrical acoustically and equally responsi've on both sides.

The improved results provided by a supporting structure embodying the invention, are illustrated by comparison of the graphs in Figures 4 and 5. Figure 4 shows a frequency response curve 50 taken from a conventional ribbon microphone in which only one frame-like plate is used to sup' port the ribbon, the plate being disposed on one side of the pole pieces. The solid black line 51 represents the response curve taken with the side of the microphone on which the ribbon support is fastened facing the sound source. The broken black line 52 represents the response curve taken with the opposite side of the microphone, on which there is no supporting plate,- facing the sound source. It will be observed that in the frequency region beyond 10,000 C. P. S. that the response curves do not agree, thus indicating the microphone is acoustically unbalanced in the high frequency region.

igure 5 is a graph showing a frequency response curve 53 taken from a similar microphone to that used for the response curves shownin Figure 4:, but constructed in accordance with the present invention wherein open ribbon supporting frames are symmetrically disposed on opp site sides of the pole pieces. The solid black line 54 represents the response curve taken with the sound source on one side of the microphone, and the broken line 55 represents the response curve taken with the sound source on the opposite side of the microphone. It will readily be seen from Figure 5 that the response curves practically co.-

incide throughout the useful range of frequencies thus indicating that the microphone. is balanced: acoustically.

While a present preferred form of the invention has been shown and described in connection with a known form of ribbon microphone, it should be understood that the invention is not limited thereto but may be applied to other forms of ribbon microphones for improving the frequency response thereof.

What is claimed is:

1. In a microphone having a magnetic field structure comprising a pair of elongated pole pieces mounted in spaced relation to provide an air gap therebetween, a vibratile member mounted in said air gap for movement in response to sound waves impinging thereon from both sides thereof, and a frame-like structure for mounting said member in said'air gap, said structure comprising a pair of open frames disposed symmetrically in spaced, parallel relation on opposite sides of said air gap and parallel to the common plane of said pole pieces, at least one of said frames being carried by and secured to an exterior surface of each of said pole pieces, and clamping means at opposite ends of said structure for engaging and holding the ends of said vibratile member in spaced relation to and between said frames thereby to support said. member within said air gap, whereby uniform acoustical impedance to said sound waves is provided with respect to both sides of said vibratile member.

2. A microphone according to claim 1, characterized in that said frames arealso disposed in planes parallel to and at equal distances from the opposite sides of said vibratile member.

3. A microphone according to claim 1, characterized in that said pole pieces are provided wtih longitudinal grooves for receiving and positioning said pair of frames, thereby to provide closer spacing of said frames and a reduction in the acoustical path length through said structure from side to side.

4. A microphone according to claim 1, characterized in that said means for clamping the ends of said vibratile member is located externally of said air gap at opposite ends thereof, and further characterized in that said frame-like structure is made of non-magnetic material.

5. In a pressure gradient type microphone, a field structure having a pair of elongated pole pieces disposed in spaced, parallel relation to provide an air gap therebetween, an elongated vibratory member disposed in said air gap for movement therein in response to sound waves impinging thereon from both the front and back thereof, a structure for supporting said vibratory member, said structure including frame-like members disposed symmetrically on opposite sides of said vibratory member in spaced relation thereto and externally of said air gap, said frame-like members being securely attached to the exterior surfaces of said field structure, and means carried by said frame-like members for supporting said vibratory member at opposite ends thereof, the symmetrical disposition of said frame-like members relative to said vibratory member providing uniform and substantially equal frequency response for both sides of said microphone.

6. A pressure gradient type microphone according to claim 5, characterized in that said supporting means is arranged for adjustably mounting said vibratory member with respect to said air gap.

7. In a pressure gradient type microphone, a field structure having a pair of elongated pole pieces disposed in spaced, parallel relation to provide an air gap therebetween, an elongated vibratory member disposed in said air gap for movement therein in response to sound waves impinging thereon from both the front and back thereof, a frame-like structure of non-magnetic material comprising two open frames disposed in i1 superposed relation symmetrically on opposite sides of said vibratory member in spaced relation thereto and externally of said air gap, said frame-like structure being securely attached to the exterior surfaces of said field structure, and

means carried by said frames for supporting said vibratory member at opposite ends thereof, whereby the frequency response of said microphone is uniform and substantially equal on both sides thereof.

8. In a velocity type microphone comprising a field structure having a pair of pole pieces disposed in spaced relation to provide an air gap, and an elongated ribbon diaphragm suspended for vibratory movement in said air gap, a framelike support for said diaphragm comprising a pair of fiat, open frames having their adjacent ends connected so that said plates will be in superposed, spaced relation, claimping means at the ends of said frames for clamping the ends of said diaphragm thereby to suspend said diaphragm within the space formed by said framelike support, and means for mounting said diaphragm and said support on said pole pieces to center said diaphragm in said air gap and to dispose said plates on opposite sides of said pole pieces symmetrically, whereby uniform and substantially equal frequency response is provided on both sides of said ribbon diaphragm when sound waves impinge thereon from either side thereof.

9. In a velocity type microphone, the combination of a magnetic field structure including a pair of elongated pole pieces mounted in spaced relation to provide an air gap therebetween, a ribbon diaphragm mounted in said air gap, and a supporting structure carried by said pole pieces for maintaining said diaphragm in said air gap. said structure comprising a pair of open frames mounted in spaced relation, the adjacent ends of said frames having extensions forming jaws for clamping the ends of said diaphragm therebetween.

10. A velocity type microphone according to claim 9, wherein said open frames are symmetrically disposed with respect to said air gap and said ribbon diaphragm on opposite sides of said pole pieces.

11. In a field structure for a velocity type microphone having a pair of elongated pole pieces mounted in spaced relation to provide an air gap therebetween, and having a ribbon diaphragm supported for movement in said air gap, and a support for said ribbon diaphragm comprising a frame-like structure carried by said pole pieces, said structure comprising a pair of open frames mounted in superposed, spaced relation on opposite sides of said air gap, said frames being provided at their ends with clamping means for holding the ends of said ribbon to thereby support said ribbon in said air gap whereby uniform acoustical impedance to sound waves on opposite sides of said diaphragm is provided.

ROBERT DUNCAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,885,001 Olson Oct. 25, 1932 2,141,429 Swickard Dec. 27, 1938 2,147,137 Anderson Feb. 14, 1939 2,346,395 Rettinger Apr. 11, 1944 

